Such an output stage is currently used in charge pumps intended to control oscillators, notably in phase-locked loop demodulators. Such a demodulator, known, for example, from European patent no. 0 398 254 B1, has a signal input intended to receive a frequency-modulated signal and an output intended to supply a demodulated signal. This demodulator conventionally comprises a phase detector intended to evaluate a phase shift that exists between a signal present on the output of an oscillator and the modulated input signal, and to supply to a charge pump control signals which represent said phase difference. The charge pump is intended to supply a current on an output connected to a loop filter during time intervals defined by the control signals. The voltage present on the terminals of the loop filter which is formed by a resistive element connected in series with a capacitive element constitutes an adjusting voltage which determines the frequency of the output signal of the oscillator, said voltage further constituting the demodulated signal.
These demodulators are often used in the field of video signal reception where the modulated signals have a frequency of the order of a hundred megahertz, which makes it possible to use, for the loop filter, a capacitor which may be easily realized in an integrated circuit containing the whole demodulator. Indeed, one of the functions of the loop filter is to supply on its terminals a variable voltage component having a frequency that is considerably lower than the frequency of the modulated signal, which component is then equivalent to a DC component of said signal. If the frequency of the modulated signal has a large value, as is the case in video signals, the time constant of the loop filter defined as being the product of the values of the resistive and capacitive elements may be chosen to be relatively low, which permits the use of a capacitor having a value lower than several picofarads.
A phase-locked loop demodulator may be used in a radio telephone but in that case the frequency of the modulated signal is much lower, for example, of the order of a hundred kilohertz for architectures of the conventional double heterodyne type, used, for example, in devices satisfying the AMPS or CT0 standards. This leads to the necessity of using a capacitor of a much higher value for the loop filter. In the state of the art it is not possible to implement in integrated form inexpensive capacitors of more than several hundred picofarads. However, it is desirable to implement the whole demodulator in integrated form. Indeed, any external component is costly by itself, calls forth the addition of extra pins on the housing containing the rest of the demodulator and gives rise to an additional assembly step during the manufacture of the radio telephone, during which the external component is to be connected to the integrated circuit. In addition, the reduction of the bulkiness of the elements forming the radio telephone makes it possible to reduce the size and the weight of this radio telephone, which offers an advantage for its user and thus forms an unavoidable necessity.
The value of the capacitor used for the loop filter depends on various parameters, among which the stability and the gain of the circuit assembly formed by the demodulator, as well as the value of a current called output current produced by the charge pump. Thus, it is possible to use a capacitor having a value of ten picofarads if the output current of the charge pump has a nominal value lower than 10 microamperes.
In the state of the art, the charge pumps are mostly used for supplying output currents which have a nominal value of the order of one milliampere. The low nominal value required in the present case, so as to enable a complete integration of the demodulator is the cause of additional problems. Indeed, the transistors forming the output stage of the charge pump have intrinsic parasitic capacitances. When the switch switches, these capacitances are charged or discharged and thus generate, on the output of the charge pump, parasitic currents which have an amplitude which is 10 to 100 times higher than the nominal value of the output current of the charge pump. These charge/discharge phenomena of the parasitic capacitances will thus considerably disturb the operation of the demodulator.
It is an object of the present invention to enable the implementation in a completely integrated form of a phase-locked loop demodulator for radio telephony applications, by proposing an output stage for a charge pump in which the influence of the charge/discharge phenomena of the parasitic capacitors on the output current of the charge pump is minimized.